The present invention relates generally to engine cooling systems and, more particularly, to a closed loop cooling system having multiple cooling loops, each having a different operating temperature.
In general, during fuel combustion in an internal combustion engine, a considerable amount of heat is generated. While the engine is designed to operate at relatively high temperatures, operating at excessive temperatures for extended periods of time is detrimental to engine efficiency and, if unaddressed, can shorten the operating life of an engine. Additionally, operating at temperatures below a desired operating temperature can have just as adverse consequences. For example, operating at too low a temperature can increase soot and condensation buildup in the engine, increase emissions, and reduce fuel efficiency. Therefore, a cooling system is provided to circulate coolant around the cylinders of the engine to provide cooling and maintain a desired operating temperature.
In outboard motors, the engine cooling fluid is often drawn from the body of water the watercraft is operated in. These types of cooling systems, that use the body of water as a reservoir, are often referred to as open loop cooling systems. That is, the coolant flow is not recirculated through the cooling system but continually draws in fresh water and discharges heated water. While this construction serves many needs satisfactorily and allows for a relatively simple construction of the cooling system, such cooling systems do have drawbacks.
One drawback to the open loop cooling system is that the quality of the coolant circulated through the internal passageways of the engine is variable. While a screen can be placed over the inlet to such a system, water born particulates can still be carried to the internal passages of the engine where the particulates can become lodged and obstruct coolant flow therethrough. Such obstruction hinders cooling of the engine in the vicinity of the blockage and can result in localized “hot-spots” during engine operation. These hot-spots are detrimental to engine performance and can result in premature engine failure if left unaddressed. Decreasing the screen openings to further limit the ingress of contaminates only promotes screen blockage and hinders adequate coolant passage.
Additionally, in watercraft operated in saltwater environments, circulating saltwater in the internal passages of the engine has its own drawbacks. Over time, salt can accumulate within the engine passages and insulate the coolant from the engine thereby hindering effective heat transfer.
Engines operated in saltwater environments with open loop cooling systems experience another adverse effect associated with the saltwater cooling flow therethrough. The flow of saltwater through the internal passages of the engine can also lead to galvanic corrosion in interior cooling passages of the engine as the saltwater flows across components manufactured from unlike materials. During galvanic corrosion, an electrolytic reaction occurs between two components manufactured from unlike materials. The saltwater acts as an electrolyte in the galvanic reaction and facilitates the corrosion of an otherwise stable component. To prevent this, manufacturers typically must include a sacrificial anode or implement other expensive manufacturing techniques.
Another drawback to open loop cooling systems in outboard motors is that the engine cannot be operated outside of a body of water. As such, servicing an engine constructed to be cooled with an open loop cooling system requires a water reservoir during operation of the engine in order to provide adequate cooling thereto. As such, having the lower portion of the outboard motor disposed in a tank of water restricts access to those systems of the motor disposed below a waterline, restricts serviceability to specific locations, and increases service time.
Furthermore, the internal combustion engine is not the only component that requires cooling during operation. Auxiliary components such as an electronic control unit (ECU), a fuel vapor separator, and an electronic regulator/rectifier also benefit from being cooled. The cooling paths to these components are even more susceptible to the detriments of open loop cooling discussed above because of smaller diameter of the coolant loop passages. While these components generate enough heat to require some cooling, they generally operate at temperatures that are lower than the preferred operating temperature of the internal combustion engine. In other words, because the internal combustion engine operates at a temperature that is higher than the operating temperature of the auxiliary components, it would be preferable to cool the auxiliary components by a system that operates at a temperature that is lower than the temperature of the system that cools the internal combustion engine.
It would therefore be desirable to provide a closed loop cooling system operable at different temperatures for different components.